7.icengineexhaustemissionslecture

7/31/2019 7.ICEngineExhaustEmissionslecture

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IC Engine Exhaust

Emissions

Section 7


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HCemissions from gasoline-powered vehicles include a number of toxicsubstances such as benzene, polycyclic aromatic hydrocarbons (PAHs),1,3-butadiene and three aldehydes (formaldehyde, acetaldehyde, acrolein).

Carbon dioxide (CO2) is an emission that is not regulated but is one of theprimary greenhouse gases, water vapour and methane are the others,believed to be responsible for global warming.

Pollutant Formation and Control

All IC engines produce undesirable emissions as a result of combustion,including hydrogen fuelled engines.

The emissions of concern are: unburned hydrocarbons (HC), carbonmonoxide (CO), nitric oxide and nitrogen dioxide (NOx), sulfur dioxide (SO2),

and solid carbon particulates (particulate matter).

These emissions pollute the environment (smog, acid rain) that contributeto respiratory and other health problems.


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Global Warming vs Climate Change

Global warming occurs because the greenhouse gases are transparent tothe high frequency solar radiation that heat up the earths surface but

absorb the lower frequency radiation from the earths surface.


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Atmospheric concentration of CO2 has increased by about 31% since thebeginning of the industrial revolution (mid1700s).

Carbon Dioxide and Global Warming

CO2 is a gas in earths atmosphere and is currently at a globally averaged

concentration of approximately 383 ppm by volume

About three-quarters of this is due to the burning of fossil fuel, the otherquarter is mainly due to deforestation

Transportation accounts for about 14% of global greenhouse gas emissionsand 19% of the CO2 emissions


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In the US a new law requires automakers to increase the average fueleconomy of their entire fleets by 40% by 2020 (motor vehicles would berequired to meet an average 6.7 L/100 km within 12 years). Canadian

govt will soon follow suit.

Carbon Dioxide and Global Warming


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Classic smog results from large amounts of coal burning in an area and is amixture of smoke and sulfur dioxide (London). Photochemical smog is due tochemical reaction of sunlight,NOx andHCin the lower troposphere producingairborne particles and ground-level ozone (O

3

)

During the 1940s air pollution as a problem was first recognized in the LosAngeles basin. Problem is due to the large population density, geography,

natural weather pattern and affinity to cars.

Emissions - Historical Perspective

In 1966 California introducedHCand CO emission limits for new vehicles.These limits were set nationally for vehicles in 1968 as part of Clean Air Act.

By making more fuel efficient engines and with the use of exhaust after

treatment, emissions per vehicle ofHC, CO, andNOx were reduced byabout 95% during the 1970s and 1980s.

Automobiles are more fuel efficient now (2x compared to 1970) but there aremore of them and the trend has been towards larger SUVs (e.g. Hummer,

Navigator, Escalade) as a result fuel usage is unchanged over this period.


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Photochemical Smog

Recipe for smog: sunlight (h),NO, HC

NO (small amount ofNO2) and hydrocarbons generated by combustionleads to the formation of many biological irritants


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NO2 + hvNO + O

O + O2 +M O3+MNO + O3NO2 + O2

O + H2O 2OH

Produce O,O3

Peroxylacetyl Nitrate (PAN) Production

RH - hydrocarbonR* - HC radicalR - methyl CH3

PAN CH3 C NO2O O

O

RC(O)O2NO2


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North American Emission Standards (g/mile)

* Phased in by 2009, NLEV - National Low Emission Vehicle voluntary program


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Diesel Dat e CO HC HC+ NOx NOx PM

Eur o 1 1 9 9 2 . 0 7 2 . 7 2 ( 3 . 1 6 ) - 0 . 9 7 ( 1 . 1 3 ) - 0 . 1 4 ( 0 . 1 8 )

Euro 2 , I D I 1 9 9 6 . 0 1 1 .0 - 0 .7 - 0 .08

Eur o 2 , D I 1 9 9 6 . 0 1a

1 .0 - 0 .9 - 0 .10

Eur o 3 2 0 0 0 . 0 1 0 .64 - 0 .56 0.50 0 .05

Eur o 4 2 0 0 5 . 0 1 0 .50 - 0 .30 0.25 0 .025

Eur o 5 2 0 0 9 . 0 9b 0 .50 - 0 .23 0.18 0 .005

e

Eur o 6 2 0 1 4 . 0 9 0 .50 - 0 .17 0.08 0 .005e

Pet r o l ( Gaso l ine)

Eur o 1 1 9 9 2 . 0 7 2 . 7 2 ( 3 . 1 6 ) - 0 . 9 7 ( 1 . 1 3 ) - -

Eur o 2 1 9 9 6 . 0 1 2 .2 - 0 .5 - -

Eur o 3 2 0 0 0 . 0 1 2 .30 0.20 - 0.15 -

Eur o 4 2 0 0 5 . 0 1 1 .0 0.10 - 0.08 -

Eur o 5 2 0 0 9 . 0 9b 1 .0 0.10

c - 0.06 0 .005

d ,e

Eur o 6 2 0 1 4 . 0 9 1 .0 0.10 c - 0.06 0 .005 d ,e

Va lues i n b racke ts a re con fo rm i t y o f p roduc t i on ( COP) l im i t sa - u n t i l 1 9 9 9 .0 9 . 3 0 ( a f t e r t h a t d a t e D I e n g in e s m u s t m e et t h e I D I l i m i t s )b - 2011 .01 fo r a l l mode l sc - an d N M HC = 0 . 0 6 8 g / k md - app l i cab le on l y to veh i cl es using D I eng inese - p roposed to be changed to 0 .003 g / km us ing the PMP m easurem en t p rocedure

EU Emission Standards for Passenger Cars (g/km)


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Starting 2009 ultra-low sulphur diesel (ULSD) with 15 ppm sulphur ismandatory in North America for highway vehicles. This is a criticalcomplement to the stringent new Tier II emission standards.

Regulation on Sulphur Content of Diesel Fuels

The average sulphur content in Canadian Diesel fuel in 2000 was 350

parts per million (ppm)

Since 2005 EU standards require diesel fuel to have less than 50 ppmsulphur content. In 2009 all vehicles will run on Sulphur-free 10 ppmsulphur diesel, including off-road.

EU also requires that diesel fuel have a minimum Cetane number of 48


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Ontario Drive Clean Program

In Ontario every vehicle must undergo a tail pipe emission test every otheryear to check compliance with emission regulations:

Nitrogen Oxide 984 ppm @ 3000 rpm

Carbon Monoxide 0.48% @ 3000 rpm and 1.0% @ 800 rpm

Unburned hydrocarbons 86 ppm @ 3000 rpm and 200 ppm @ 800 rpm

Particulates (diesels only at present) 30% opacity

Evaporative emissions from gas refuelling cap (SI only at present)


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Test results between 1999 and March 2004

Light-Duty Program*: 14.6% failed test

Heavy-Duty Diesel**: 4% failed test

Heavy-Duty Non-Diesel**: 27.3% failed test

* 6 million vehicles (automobiles, vans, SUVs, pick-ups) in program** 200,000 vehicles in program

Ontario Drive Clean Program Stats


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Nitrogen Oxides (NOx)

NOx includes nitric oxide (NO) and nitrogen dioxide (NO2), in SI enginesthe dominant component ofNOx isNO

NOx forms as a result of dissociation of molecular nitrogen and oxygen.

)(222 NOON +

Zeldovich mechanismO+N2NO+N

N+O2NO+O

since the activation energy (E) of the first reaction is veryhigh the reactionrate, '' ~ exp (-E/RT), is very temperature dependent

NO is only formed at high temperatures (>2000K) and the reaction rateis relatively slow.


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Since the chemistry is not fast enough the actualNO concentration tendstoward but never achieves the equilibrium value.

IfNO concentration is lower than equilibrium value NO formsIfNO concentration is higher than equilibrium value NO decomposes

Since the cylinder temperature changes throughout the cycle theNO reactionrate also changes.

SI Engine In-cylinderNO Formation

= 1

0

dxxxNONO

Each fluid element burns to its AFT based on its initial temperature, elementsthat burn first near the spark plug achieve a higher temperature.

Once the element temperature cools to 2000K the reaction rate becomes soslow that theNO concentration effectively freezes at a value greater than

the equilibrium value.

The total amount ofNO that appears in the exhaust is calculated by summingthe frozen mass fractions for all the fluid elements:


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x 0

-15o (x 0)x 1

25o (x 1)

x 0

x 1

Equilibrium concentration:based on the local temperature, pressure,equivalence ratio, residual fraction

Actual NO concentration:based on kinetics

(assuming no mixing of fluid elements)


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One would expect the peakNO concentrations to coincide with highest AFT.

Effect of Equivalence Ratio onNO Concentration

Typically peakNO concentrations occur for slightly lean mixtures thatcorresponds to lower AFT but higher oxygen concentration.


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Effect of Various Parameters onNO Concentration

Increased spark advance and intake manifold pressure both result in highercylinder temperatures and thus higherNO concentrations in the exhaust gas

= 0.97

= 1.31

= 1.27

= 0.96

Pi= 354 mm HgPi= 658 mm Hg


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ExhaustNO Concentration Reduction

Since the formation ofNO is highly dependent on cylinder gas temperatureany measures taken to reduce the AFT are effective:

increased residual gas fraction exhaust gas recirculation (EGR) moisture in the inlet air run fuel lean

IDI/NA indirect injectionnaturally aspirated

DI/NA direct injectionnaturally aspirated

In CI engines the cylinder gas temperature is governed by the load andinjection timing


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Hydrocarbons

Hydrocarbon emissions result from the presence of unburned fuel in theengine exhaust.

However, some of the exhaust hydrocarbons are not found in the fuel, but arehydrocarbons derived from the fuel whose structure was altered due tochemical reaction that did not go to completion. For example: acetaldehyde,formaldehyde, 1,3 butadiene, and benzene all classified as toxic emissions.

About 9% of the fuel supplied to the engine is not burned during the normalcombustion phase of the expansion stroke.

Only 2% ends up in the exhaust the rest is consumed during the otherthree strokes.

As a consequence hydrocarbon emissions cause a decrease in the thermalefficiency, as well as being an air pollutant.


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Hydrocarbon Emission Sources for SI Engines

There are six primary mechanisms believed to be responsible forhydrocarbon emissions:

% fuel escapingSource normal combustion %HCemissions

Crevices 5.2 38Oil layers 1.0 16Deposits 1.0 16Liquid fuel 1.2 20Flame quench 0.5 5Exhaust valve leakage 0.1 5

Total 9.0 100


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Hydrocarbon Emission Sources

Crevices these are narrow regions in the combustion chamber into whichthe flame cannot propagate because it is smaller than the quenching distance.

CrevicePiston ring

Crevices are located around the piston, head gasket, spark plug and valveseats and represent about 1 to 2% of the clearance volume.

The crevice around the piston is by far the largest, during compression the fuel

air mixture is forced into the crevice (density higher than cylinder gas since gasis cooler near walls) and released during expansion.


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Oil layers - Since the piston ring is not 100% effective in preventing oilmigration into the cylinder above the piston, an oil layer exists within thecombustion chamber that traps fuel.

Hydrocarbon Emission Sources

Deposits - Carbon deposits build up on the valves, cylinder and pistoncrown. These deposits are porous with pore sizes smaller than thequenching distance so trapped fuel cannot burn.

Liquid fuel - For some fuel injection systems there is a possibility that liquidfuel is introduced into the cylinder past an open intake valve. The less volatilefuel constituents may not vaporize (especially during engine warm-up) and beabsorbed by the crevices or carbon deposits

Flame quenching - It has been shown that the flame does not burncompletely to the internal surfaces, the flame extinguishes at a small butfinite distance from the wall.

H d b E h P


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During the exhaust stroke the piston rolls the hydrocarbons distributed alongthe walls into a large vortex that ultimately becomes large enough that aportion of it is exhausted.

Hydrocarbon Exhaust Process

When the exhaust valve opens the large rush of gas escaping the cylinderdrags with it some of the hydrocarbons released from the crevices, oil layerand deposits.

Blowdown(near BC)

ExhaustStroke

H d b E h t P


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Hydrocarbon Exhaust Process

Exhaustvalveopens

Exhaustvalvecloses

The first peak is due to blowdown and the second peak is due to vortex rollup and exhaust (vortex reaches exhaust valve at roughly 290o)

TCBC


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Undermixing of fuel and air -Fuel leaving the injector nozzle at low velocity,at the end of the injection process cannot completely mix with air and burn.

Overmixing of fuel and air - During the ignition delay period evaporated fuelmixes with the air, regions of fuel-air mixture are produced that are too lean to

Burn, some of this fuel makes its way out the exhaust longer ignition delaymore fuel becomes overmixed.

Hydrocarbon Emission Sources for CI Engines

Crevices - Fuel trapped along the wall by crevices, deposits, or oil due toimpingement by the fuel spray (not as important as in SI engines).

ExhaustHC,pp

mC

air


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Note for the direct injection diesel the hydrocarbon emission are worse atlight load (long ignition delay)


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Particulates

A high concentration of particulate matter (PM) is manifested as visiblesmoke in the exhaust gases.

Particulates are any substance other than water that can be collected byfiltering the exhaust, classified as:

1) solid carbon material or soot2) condensed hydrocarbons and their partial oxidation products

Diesel particulates consist of solid carbon (soot) at exhaust gas temperaturesbelow 500oC, HC compounds become absorbed on the surface.

In properly adjusted SI engines soot is not usually a problem

Particulate can arise if leaded fuel or overly rich fuel-air mixture are used.Burning crankcase oil will also produce smoke especially during engine warmup where the HC condense in the exhaust gas.


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Most particulate material results from incomplete combustion of fuelHCforfuel rich mixtures.

Particulates (soot)

)()2(2

2 22 sCaxHy

aCOaOHC yx +++

i.e. when the (C/O) ratio of reactants exceeds 1.Experimentally the critical C/O ratio for onset of soot formation is 0.5 - 0.8

OHOHCOOsCCOOCO 22222222

1)(

2

1+++

Any carbon not oxidized in the cylinder ends up as soot in the exhaust!

Based on equilibrium the composition of the fuel-oxidizer mixture sootformation occurs whenx2a (or x/2a 1) in the following reaction:

The CO, H2, and C(s) are subsequently oxidized in the diffusion flame to

CO2 andH2O via the following second stage


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Particulates are a major emissions problem for CI engines.

Particulates and CI Engines

= 0.7

= 0.5

= 0.3

One technique for measuring particulateinvolves diluting the exhaust gas with

cool air to freeze the chemistry beforemeasurements

Exhaust smoke limits the full load overall equivalence ratio to about 0.7


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An example of this dilemma is changing the start of injection, e.g., increasingthe advance increases the AFT

Particulates and CI Engines

Crank angle bTC forstart of injection

In order to reduceNOx one wants to reduce the AFT but that has the adverseeffect of decreasing the amount of soot oxidized and thus increases the

amount of soot in the exhaust.

C b M id


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Carbon Monoxide

Carbon monoxide appears in the exhaust of fuel rich running engines, thereis insufficient oxygen to convert all the carbon in the fuel to carbon dioxide.

C8H18-air

C b M id


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Carbon Monoxide

The C-O-H system is more or less at equilibrium during combustion andexpansion.

Late in the expansion stroke when the cylinder temperature gets down toaround 1700K the chemistry in the C-O-H system becomes rate limited andstarts to deviate from equilibrium.

In practice it is often assumed that the C-O-H system is in equilibrium until

the exhaust valve opens at which time it freezes instantaneously.

The highest CO emission occurs during engine start up (warm up) when theengine is run fuel rich to compensate for poor fuel evaporation.

Since CI engines run lean overall, emission of CO is generally low and notconsidered a problem.

Emission Control


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Three basic methods used to control engine emissions:

1) Engineering of combustion process - advances in fuel injectors, oxygensensors, and engine control unit (ECU).

Emission Control

The current emission limits forHC, CO andNOx have been reduced to 4%,4% and 10% of the uncontrolled pre-1968 values, respectively.

2) Optimizing the choice of operating parameters - twoNOx control measures

that have been used in automobile engines since 1970s are spark retard andEGR.

3) After treatment devices in the exhaust system - catalytic converter

Catalytic Converter


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Catalytic Converter

Lead and sulfur in the exhaust gas severely inhibit the operation of a catalytic

converter (poison).

The catalytic converter uses a reduction catalyst and an oxidation catalyst toremove CO, NO, andHCfrom the exhaust streamThe catalytic converter uses a reduction catalyst and an oxidation catalyst toremove CO, NO, andHCfrom the exhaust stream

Both consist of a ceramic honeycomb coated with a metal catalyst, usuallyplatinum, rhodium and/or palladium.


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Three-way Catalytic Converter

A three-way catalysts will function properly only if the exhaust gas compositioncorresponds to nearly (1%) stoichiometric combustion.

If the exhaust is too lean NO is not destroyedIf the exhaust is too rich CO andHCare not destroyed


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Three-way Catalytic Converter

Since thermal efficiency is highest for slightly lean conditions it may seem thatthe use of a catalytic converter is a rather severe constraint.

The same high efficiency can be achieved using a near stoichiometric mixtureand diluting with EGR to reduce NOx

Reduction catalyst:In the first stage platinum and rhodium are used to removeNOx. TheNO

molecule dissociates on the catalyst surface producing molecular oxygenand nitrogen that are released

2NO N2 + O2 or 2NO2 N2 + 2O2

Oxidation catalyst:In the second stage platinum and palladium are used to oxidize the COand the unburned hydrocarbon (HC) using the oxygen in the stream.

2CO + O2 2CO2

2CxHy + (2x+y/2)O2

2xCO2 + yH2O

Effect of Temperature


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Effect of Temperature

The temperature at which the converter becomes 50% efficient is referred toas the light-off temperature.

The converter is not very effective during the warm up period of the engine

Emission Control


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Wide-band sensor output is linear and canbe used to measure the O2 in the gas stream.Is used for tuning, only used in a few vehicles

Emission Control

A closed-loop control system with an oxygen (lamda) sensor in the exhaust isused to control the fuel delivery so that the A/F ratio is near stoichiometric.

Bosche LSU-4 wide band sensor

The narrow-band oxygen sensor when hot (800oC) produces a voltage thatvaries according to the amount of oxygen in the exhaust compared to theambient oxygen level in the outside air.

Sensor output is very nonlinear ranging from 0.2 VDC (lean) to 0.8 VDC (rich),

a stoichiometric mixture gives an average reading of around 0.45 Volts.

The sensor can contain a heater to bring it quickly up to temperature and islocated before the catalytic converter


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Diesel Particulate Filters (DPF) are used for removing PM.Active DPFs: raise temperature of the filter by periodically adding fuel to theexhaust stream that combusts in the filter raising the DPF temp cleans theDPF by oxidizing the collected PM with O2, requires >600

oC (regeneration).

Diesel engines run fuel lean (reduce soot) so a 3-way catalytic converter isnot useful, also particulate matter (PM) consisting of Cneeds to be removed.

Diesel Exhaust Treatment

Johnson Matthey CRT

Non catalyst

(reaction requires > 250o

C)

Oxidizer catalysts used for reducingHCand CO

Passive DPF:


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(NH2)2CO HNCO + NH3HNCO + H2O CO2 + NH3

Selective Catalytic Reduction (SCR) used to convertNOx intoN2 andH2O.

4NH3 + 4NO + O2 4N2 + 6H2O

2NH3 + NO + NO2 2N2 + 3H2O

8NH3 + 6NO2 7N2 +12H2O

Typically an aqueous solution of urea (NH2)2CO is added to the exhaust

stream to produce ammonia:

Diesel Exhaust Treatment

NO can be reduced by retarding fuel injection from 20o to 5o before TC in orderto reduce the peak combustion temperature at the expense of efficiency.

In a lean burn engine, it is necessary to add a reductant like ammonia (NH3)to the gas stream to enable this reaction over a catalyst.

Mercedes-Benz BlueTEC ML320 has a 7 gal urea based Addblue tank

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